1. Field of the Invention
The present invention is directed to a high-strength linepipe and structural steel that is resistant to hydrogen-induced cracking (HIC) in sour service.
2. Description of Related Art
A continuing need exists to develop steels having high strength which can provide extended service life as linepipe in sour gas (H.sub.2 S) service. High strength linepipe for this sour service has heretofore been produced from low carbon-manganese steel, and strengthened by the addition of niobium and/or vanadium. Manganese levels for such steels have typically been in the range of 0.90 to 1.20 weight percent, when it is expected that the linepipe will be used in the most severe service conditions. For the purposes of this disclosure, manganese levels in the aforesaid range of 0.90-1.20 weight percent will be referred to as being "relatively high" manganese contents for low carbon-manganese steels.
While providing resistance to cracking due to exposure to sour gas, these steels are prone to manganese sulfide stringer formation, due to the relatively high level of manganese employed in the steel. This is the case even where the steel has very low sulfur levels (&lt;0.003 wt. percent), because the Mn:S ratio is very high (&gt;40,000:1). In order to combat this tendency to form manganese sulfide stringers, the inclusion of calcium, which causes preferential formation of globular or angular calcium oxysulfide inclusions, has become the standard practice. Rare earth metals have also shown the ability to reduce the tendency of the steel to form manganese sulfide stringers. However, both calcium and rare earth additions are expensive and can give rise to processing difficulties such as generation of excessive fumes, nozzle blocking or poor cleanliness ratings.
In casting linepipe steel, from a processing standpoint, steels having manganese contents above 1.0 weight percent are also prone to centerline segregation when casting speeds are high. Further, centerline segregation can occur when proper superheats are not maintained and/or when machine maintenance and water cooling practices are poor.
It is therefore a principal object of the present invention to provide a high strength steel which is suitable for extended use in wet, sour gas service.
It is a further principal object of the present invention to provide a high strength steel having a very low manganese content, yet which is resistant to sour gas (H.sub.2 S) degradation.
It is an additional important object of the present invention to provide a high strength steel composition, suitable for sour gas service, which can be continuously cast at the high, normally desired, speeds employed in casting non-linepipe steel compositions.
It is a further important object of the present invention to provide a high strength steel composition that avoids the need to treat the alloy with calcium or rare earth metals in order to reduce the formation of manganese sulfide stringers.
It is an additional object of the present invention to provide a high strength, high toughness steel that is remarkably resistant to stepwise cracking and to sulfide stress cracking.
It is a further object of the present invention to provide a high-strength steel that has a very low carbon and manganese content as compared to high strength steels currently used in sour gas service.